1. Field of the Invention
The present invention relates to a light emitting device using silicon, and more particularly, a laser using ultrathin silicon.
2. Description of the Related Art
An optical communication is adopted in a broadband network supporting an Internet industry. A laser using a compound semiconductor of the III-V group or II-VI group is used for transmission and reception using light in such an optical communication.
Various structures are proposed for the compound semiconductor laser, but a double hetero structure is generally used. In the double hetero structure, a compound semiconductor having a small band gap is fitted in a compound semiconductor having a large band gap by using two different kinds of compound semiconductors. In order to fabricate the double hetero structure, an n conductive type compound semiconductor, an undoped i conductive type compound semiconductor, and a p conductive type compound semiconductor are laminated on a substrate in a vertical direction by continuous epitaxial growing. Meanwhile, it is important to consider a band structure of the undoped i type compound semiconductor interposed between the n-type and p-type compound semiconductors. It is important that the band gap of the i-type compound semiconductor is smaller than that of the n-type or p-type compound semiconductors, a conduction band level of the i-type compound semiconductor is lower than that of the n-type compound semiconductor, and a valence band level of the i-type compound semiconductor is higher than that of the p-type compound semiconductor. That is, electrons and holes are trapped in one area of the i-type compound semiconductor. Accordingly, since the electrons and the holes are easy to exist on the same area, there is high probability that the electrons and the holes will be annihilated by collision increases, thereby improving luminous efficiency. Since a refractive index tends to increase as the band gap decreases, the light is also trapped in the i-type compound semiconductor by selecting a material having the refractive index of the i-type compound semiconductor smaller than that of the n-type or p-type compound semiconductor. Since the trapped light efficiently induces rebinding of the electrons and the holes which form inverted distribution, a laser beam is oscillated.
A long-range data communication is instantly made in a mass by the optical communication using the compound semiconductor having such excellent luminescence efficiency. That is, processing or storing of data is performed on an LSI using the silicon, while transmission of the data is performed by the laser using the compound semiconductor.
If the silicon may be emitted at high efficiency, it is possible to integrate both an electronic device and a light emitting device on a silicon chip, resulting in a valuable industrial technology. Therefore, a lot of researches for making the silicon emit the light have been conducted.
However, it is difficult to make the silicon emit the light at high efficiency since the silicon has an indirect transition type band structure. The indirect transition type band structure is a band structure in which any one of the momentum of which the energy of the conduction band is at a lowest level and the momentum of which the energy of the valence band is at the lowest level is not 0. In case of the silicon, the minimum energy point of the valence band is a point Γ where the momentum is 0, while the minimum energy point of the conduction band is not the point Γ but exists between the point Γ and a point X. More specifically, when a grating constant is represented by ‘a’ and k0=0.85*π/a is defined, the minimum energy point degenerates and exists at six points of (0, 0, ±k0), (0, ±k0, 0), and (±k0, 0, 0).
Since the minimum energy points of both the conduction band and the valence band exist at the point Γ in most compound semiconductors, the compound semiconductors are called a direct transition type semiconductor.
Next, the reason why the luminescence efficiency is low in an indirect transition type semiconductor and the reason why the luminescence efficiency is high in a direct transition type semiconductor will be described.
As described above, in order to allow a semiconductor element to emit the light, the electrons and the holes are annihilated due to the collision causing the difference in energy between both sides to produce the light. At this time, both an energy conservation law and a momentum conservation law should be satisfied. The electrons have an energy level in the conduction band and the holes have an energy level in the valence band without the electrons. The difference between both sides is the energy level of the light. Since a wavelength is different for each energy level, the difference in the energy level between the conduction band and the valence band, that is, the size of the band gap determines the wavelength of the light, that is, a color. Therefore, there is no particular difficulty in satisfying the energy conservation law.
Meanwhile, the collision of the electrons and the holes causes the light to be emitted, thus the momentum should be conserved. By quantum mechanics which is a law controlling a microscopic world, the electrons, the holes, and photons (quantum of light) are represented as waves, but since they are dispersed as particles, the momentum conservation law is satisfied. The momentum is a scale that quantifies the degree of power the particles are qualitatively flickered in collision of the particles. In consideration of the dispersion relationship of the light ω=ck (where, ω represents an angular frequency of the light, c represents high speed, and k represents the momentum of the photon) and the energy of the light, the momentum of the photon in crystal is approximately 0. Even though the particles are flickered as the light collides with the particles, a material is little dispersed by flickering of the particles with our intuitions.
Meanwhile, since the hole also has the minimum energy point at a point Γ, the hole has almost little momentum. However, since the electron almost never exists at the point Γ in the silicon which is the indirect transition type semiconductor and exists at a minimum energy point in the vicinity of a point X, the electron has a large momentum of k0=0.85*π/a.
Accordingly, if the silicon is used, both the momentum conservation law and the energy conservation law cannot be satisfied when the electrons and the holes just collide with each other. Herein, phonon which is the quantum of photon oscillation in the crystal is absorbed or discharged, and only an electron-hole pair capable of satisfying both the momentum conservation law and the energy conservation law anyhow is transformed to the light. Even though such a process may be physically present, an environment in which the electron, hole, photon, and phonon collide with each other at the same time is a sophisticated dispersion process, there is very low probability that such an environment will occur. Therefore, it is known that the silicon which is the indirection transition type semiconductor is very low in luminescent efficiency.
Since most of the direct transition type compound semiconductors have the minimum energy point at a point Γ in both the conduction band and the valence band, both the momentum conservation law and the energy conservation law can be satisfied. Accordingly, the compound semiconductors have high luminescent efficiency.
Non-patent Document 1 (pp. 143508-1-143508-3 of Vol. 88 in 2006, “Applied Physics Letters” written by R. Chan, M. Feng, N. Holonyak, Jr., A, James, and G. Walter) discloses a transistor laser device which drives a laser using a compound semiconductor having the high luminescent efficiency in a bipolar transistor made of the compound semiconductor.
As described above, even though the silicon is very low in luminescent efficiency in a bulk state, the luminescent efficiency of the silicon increases by changing the bulk state into a porous state or a nanoparticle state.
For example, Non-patent Document 2 (pp. 1046-1048 of Vol. 57 in 1990, “Applied Physics Letters” written by L. T. Canham) discloses a report that when silicon anodized in a fluorinated acid solution is in the porous state, the silicon emits light in a visible light wavelength band at room temperature. Even though such a mechanism is not completely defined, it is believed that a quantum size effect occurring to allow the silicon trapped in a narrow area due to formation of a porosity to exist may be important. In case of the silicon having a small size, since the electron is trapped in the narrow area, the momentum cannot be adversely determined according to an indeterminacy principle of the quantum mechanics. Therefore, it is thought that the electron and the hole can be easily be recoupled.
As another method using the silicon, Non-patent Document 3 (pp. 2077-2079 of Vol. 69 in 1996, “Applied Physics Letters” written by S. Coffa, G. Franzo, and F. Priolo) discloses a report that a light emitting diode is fabricated which becomes a light emitting element by injecting an Er ion into a p-n junction formed on an Si substrate. When the Er ion is injected into the Si substrate, the Er ion forms an impurity level. Since the impurity level is a spatially localized level, the momentum of the electron is substantially 0 when the electron existing in an Si conduction band is trapped at the impurity level formed by the Er ion. Therefore, the electron is recoupled with the hole in the valence band, thereby emitting the light. Since the light emitted via the Er ion has a wavelength of 1.54 μm, the light may be propagated without being absorbed in adjacent silicon. Further, light that is emitted by using the existing optical fiber has a wavelength with decreased loss, the existing optical fiber network may be used even in a case that a Si based LED using the Er ion is practically used by a future technological innovation. Therefore, it is expected that a large-scale equipment investment is not required.
As another method using the silicon, Non-patent Document 4 (pp. 8354-8356 of Vol. 89 in 2001, “Applied Physics Letters” written by F. Iacona, G. Franzo, E. C. Moreira, and F. Priol) or Non-patent Document 5 (pp. 44-49 of Vol. 89 in October 2005, “IEEE Spectrum” written by S. Coffa) disclose a report that it is possible to more efficiently emit the light by injecting the Er ions into silicon nanoparticles by combining the quantum size effect with the Er ions.
A high speed data communication is implemented by connecting the inside of the silicon chip or the silicon chips to each other through a waveguide, etc., and using the light, emitting the light only with the LED is insufficient and thus it is necessary to fabricate a laser diode (LD) having excellent monochromaticity, straight advancement property, and coherency, and having intensity or a phase which may be modulated at high speed.
In order to fabricate the LD, it is necessary to combine a light source composed of a p-n diode, etc., the waveguide, and a mirror with each other.
For example, JP-A-2004-319668 discloses a device for oscillating a laser by combining a waveguide type diffraction grating with a p-n junction added with a rare-earth element. In this device, emission of the light from the rare-earth element is used.
Further, JP-T-2002-536850 discloses a device for oscillating the laser by combining a layer of mirrors vertically deposited on a substrate with a light emitting diode using the silicon. In this device, a p-n junction of a light emitting unit is formed perpendicular to the substrate.
However, as described above, in a case when the silicon is used as the main material for the light source, the luminescent efficiency is insufficient, whereby it is difficult to oscillate the laser.
Therefore, as a method of forming the LD on the silicon chip, for example, Non-patent Document 6 (pp. 1143-1145 of Vol. 18 in 2006, “IEEE Photonics Technology Letters” written by A. W. Fang, H. Park, R. Jones, O. Cohen, M. J. Paniccia, J. E. Bowers) discloses a method of fabricating a resonator constituted by the compound semiconductor as the light source, the waveguide, and the mirror including the silicon. This device uses the compound semiconductor as the light source and has a hybrid structure using the silicon as the resonator, thereby oscillating the laser by overlapping evanescent permeation of the light trapped in the silicon with the compound semiconductor serving as a gain medium.
Here listed U.S. patent application Ser. No. 11/935904 filed by the present applicant as Prior Application related to the present application.